Electric insulating material and to conductors insulated with such material



arch 10 1&94, A. A, NEW

ELECTRIC INSULATING MATERIAL AND TO CONDUCTRS n INSULATED WITH SUCH MATERIAL Filed Feb. 11, 1938 IN1/Emol? Aem/@fem ALA/v MEW B Y' f TTORNE Y Patented Mar. 10, 1942 2,276,084 lcs ELECTRIC INSULATI'NG MATERIAL AND TO CONDUCTORS INSULATED WITH SUCH MATERIAL Archibald Alan New, London, England, assigner to International Standard Electric Corporation, New York, N. if..

Application February 11, 1938, Serial No. 190,031

In Great Britain February 20, i937 Z Claims.

Esters of cellulose have frequently been proposed for use as electric insulating materials, both in the form of threads spun from such esters and in the formrof native cellulose such as cotton or paper that has been partially esteriiied.

In British Patents 372,312 and 395,737 there 'are described the partial esterication of cotton and paper up to the mono-acetate stage or in some cases up to the di-acetate stage. Such partially esteried materials possess a comparatively slight increase in electric resistance under dry conditions but a very considerable increase in electric resistance under humid conditions when measured with direct current and compared with the resistance of the corresponding unesterified cellulosic materials.

Cellulose that has been esteried up to the mono-acyl stage was found to show a great -improvement under humid conditions in power factor when measured with alternating current of power frequencies. This improvement, however, rapidly decreased with increase of frequency as will'be clear from the following table showing the power factor of different forms of cellulose at different frequencies. All the figures relate to cellulose yarns at 60% relative humidity and a y temperature ofv C.

It will be noted that the improvement in power factor of cotton having 55% combined acetic acid content over cotton having 29% combined acetic acid content is much' less ai; 1000 cycles than at 50 cycles, so that it would be expected that at (Cl. 17d- 29) currents having a frequency of l0 kc. and above radio frequencies little or no improvement with respect to the original well Washed cellulose would be obtained.

l 400 800 1000 cycles cycles cycles cycles Washed cotton 0.003 0.005 0.006 O. 6 Cotton having 20% combined 4 OO acetic acid content 0.003 0.005 0.005 0.00.55 Cotton having combined acetic acid content 0.003 0.004 0.005 0.005

These figures heightened the impression that under dry conditions no improvement at all in power factor for radio frequencies would be secured by acetylating to a high degree.

The increase in electric resistance to direct current obtained by esterifying the cellulose beyond the diacyl stage is not generally sufficiently great to justify the process being carried to that extent, but for special purposes the higher values of direct current resistance under humid conditions that may be obtained by esterifying beyondv the diacyl stage have led to such processes being carried out even up to the stage of full esteriiication. For these processes it is necessary to add to the bath an inert diluent such as toluene to prevent excessve swelling of the material and tendering of the yarn.

With the development of the transmission of higher and higher frequencies over electric cables it has become necessary to use electric insulating material thathas high electric resistance and low power factor at higher frequencies than have hitherto been taken into consideration. It isv only recently, however, that measurements of A. C. resistance and power factor at frequencies higher than about l kc. could be carried out With accuracy, particularly on yarn materials.

The single gure of the drawing shows a c oaxialcable insulated in accordance with my invention.

According to this invention we provide an electric conductor for the transmission of electric (whichk may be superimposed on D. C. currents or on A. C. currents of lower frequency) insulated by native cellulosic material that has been esterifled up to and beyond the diacyl stage without loss of tensile strength sufficient to prevent the material from being applied to the' conductor in the manner ln which it is usually applied, including native cellulosic materials that have been fully esteried without 4such loss of 4tensile strength.

For the purpose of this invention the only cellulose esters to be considered are those of carboxylic acids having from 2 5 carbon atoms, as these are the esters that We have so far found suitable, but the invention must be taken to include mixed cellulose esters of such acids.

2-5 carbon atoms prepared by esterifying native cellulose up to and beyond the diacyl stage without appreciable tendering of the fibres possess very low power factors for the higher frequencies of alternating current and that the increase of Ais only 0.005.

cycles was '1.0 decibels per mile.

compared with that of the lower esterified materials. The power factor of acylated cotton yarn having 55% combined acetic acid content for example, increases from 0.0055 at 1' kc. to 0.008'at 1000 kc. under conditions of ordinary atmospheric humidity whilst the power factor of acetylated cotton yarn having 29% combined acetic acid content increases under similar conditions from 0.009 at 1 kc. to 0.015 at 1000 kc. Moreover; contrary to previous experience with lower combined acetic contents, a large improvement in power factor at 1000 kc. even under dry conditions has been obtained by esterification of cotton tok a combined acetic acid content of- 55% and above. Thus the power factor of acetylated cotton yarn having 29% combined acetic acid content is 0.013 for 1000 kc. under dry conditions, but that of cotton yarn acetylated to 55% acetic acid content Also, under dry conditions the power factor of cotton acetylated to 55% is nearly constant between 1 kc. and 1000 kc., only rising from .0045 to .0055 over this wide frequency range. y

By carrying the acetylation still further to a combinedl acetic acid content of 60 to 62% a still lower power factor and a flatter frequency power factor characteristic can be obtained, i. e. at 1 kc. 0.004 and at 1000 kc. 0.0043.

The present invention is of particular value when applied to coaxial conductor electric cables for the transmissionof a very wide frequency band, and more especially to cables constructed as shown in U. S. patents to Wentz Nos. 2,018,477 issued October 22, 1935 and 2,038,973, issued April 28, 1936 and in the figure of the drawing in which, according to the present invention, the insulating separator or separators is or are composed of native cellulose esterified to or beyond the diacyl stage. By the use of such material for the separator wevhave been able to raise the upper limit of the frequency band satisfactorily transmitted in this type of cable as compared with cables wherein lower esterified cellulose has been used as the separator. For example, in a certain type of coaxial cable insulated with cotton acetylated -to 29% combined acetic acid content, the attenuation for a frequency .of 21/2 mega- When insulation consisting of cotton acetylated to 55% combined aceticV acid content was used, the attenuation for this frequency was 5.9 decibels per mile.

In the preparation of highly esteriiied cellulose several different catalysts Yhave different effects upon the electric insulating properties of the final product but these effects are not of such great importance with respect to the properties in relation to alternating current under dry or moderately humid conditions as they are 'with respect to the resistance as measured with direct current. For some purposes however, it is desirable to use as the insulating material for electric cables material that has not onlyy a high electric resistance and low power factor when measured with alternating current but also a high electric resistance to direct current under. humid conditions. For these purposes additional precautions, such as are described in the specification of British Patent 489,928 by Lovell r..-" trange Eaton Ellis as a communication from Chemical Works formerly Sandoz, must be taken in the esterication process. In particular we have found that when sulphuric acid is used as a catalyst, not only is there a decrease in tensile strength of the esterifled mapower factor with increase of frequency is small terial, but the direct current resistance under humid conditions is very much lower, compared in each case with similar material for the preparation of. which the zinc chloride type of catalyst is used. The alternating current power factors under dry and moderately humid conditions, on the other hand, are equal to that of material using the zinc chloride type of catalyst. Under conditions of high humidity, however, even the alternating current power factor is adversely affected, and accordingly for use in such conditions it is desirable that material prepared with,

sulphuric acid as a catalyst should be avoided.

I Example 1 The constitution of the bath for the preparation of lmaterials to be used with electric ccnductors according to the invention can bez- Percent Toluene, petrol or other inert diluent 50 Acetic anhydride 25 Acetic acid 20 Zinc chloride 5 Example 2 Cotton yarn or other cotton material purified by bucking and bleaching is carefully washed, then dried to a moisture content of about 1-2% and immersed for two hours at 40 C. in acetic acid containing about 2% of water, then taken out, pressed to 60% of adherent liquid and then immersed in the following bath:

Percent Benzene, petrol or other inert diluent 65 Acetic acid anhydride 15 Glacial acetic acid 15 Anhydrous zinc chloride 5 The acetylation is carried out so as to allow the circulation of the acetylating liquor through cellulosic material for 25 hours, at a temperature of 40 C., which in the case of cotton yarn purifled as above gives a combined acetic acid content 01' 56%. After this time the acetylated yarn is carefully washed, the degree of washing being controlled by-measuring the electrical conductivity of the wash water and finally hydro-extracted and dried at about C.

I claim:

1. An i electric conductor forming part of a cable for transmission of frequencies of l0 kc.

or over, and insulation for said conductor having sufficient tensile strength to permit the material to be applied in the usual manner, and having a power factor less than .01 at the fre- `quencyrange specified, comprising native cel- -lulosic material esteried at least to the diacyl stage'to form an ester or esters of a carboxylic acid or acids having from two to five carbon atoms.

2. Electric conductor as claimed in claim l in which the esterifled cellulosic material is free from ions derived from sulphuric acid.

ARCHIBALD ALAN New. 

